1. Field of the Invention
This invention relates to a method for preparing aluminum nitride (AlN) ceramics, especially for preparing AlN ceramics having high thermal conductivities.
2. Description of the Prior Art
With the advances of high integration density and high output power of the recent semiconductor integrated circuits, the value of power consumption per unit area of a semiconductor chip is increasing. Unless the generated heat is sufficiently radiated, the integrated circuit produces a maloperation or eventully breaks down. The insulating substrates to mount integrated circuit chips, which have larger thermal conductivities have been demanded.
Heretofore, alumina ceramics has been widely used as the materials for the insulating substrates to mount semiconductor chips. However, as the power dissipated in a semiconductor integrated circuit increases, the thermal conductivity becomes to be insufficient. The insulating substrate having more improved thermal conductivity is still demanded. For the material of such insulating substrate, a large electrical resistance, a thermal expansion coefficient close to that of silicon and a high mechanical strength are required in addition to the high thermal conductivity.
As the material satisfying these requirements, the use of aluminum nitride ceramics has recently proposed. The aluminum nitride ceramics has a thermal expansion coefficient of about 4.3.times.10.sup.-6 /.degree.C. (average from room temperature to 400.degree. C.) which is smaller than the thermal expansion coefficient, about 7.times.10.sup.-6 /.degree.C., of the alumina ceramics and which is close to the thermal expansion coefficient, 3.5 to 4.0.times.10.sup.-6 /.degree.C., of silicon. The mechanical strength of the aluminum nitride ceramics is about 50 Kg/mm.sup.2 of a flexural strength and is strong compared to the alumina ceramics having that of 20 to 30 Kg/mm.sup.2.
The aluminum nitride ceramics is prepared by sintering aluminum nitride powder. However, since the aluminum nitride is covalent bond material, it is difficult to obtain densified ceramics. To sinter the aluminum nitride powder, appropriate sintering additives are used. Japanese published unexamined patent applications (TOKKAISHO) Nos. 54-100410, 50-23411 and 58-55377 use oxides such as calcium oxide (CaO), barium oxide (BaO) and strontium oxide as the sintering additives. However, those oxides do not cause reducing reaction. According to the findings of the present inventors, oxygen contained in the aluminum nitride powder deteriorates the thermal conductivity of the resultant aluminum nitride ceramics, and the use of oxides cannot remove the oxygen content to restrict improving the thermal conductivity. For example, when calcium oxide (CaO) is added to aluminum nitride powder with a content of 0.6 wt % and the mixture is sintered under a sintering temperature of about 1800.degree. C. for two hours, the obtained aluminum nitride ceramics has a thermal conductivity between 40 to 50 W/mK. The best value of the thermal conductivity reported in the above-mentioned Japanese published unexamined patent applications were at most 70 W/mK.